1. Field of the Invention
The present invention relates to a projection type display apparatus utilizing a spatial light modulator (light valve), and more particularly, to a projection type display apparatus having the structure for securing optically stable performance against influence of external stress and effectively restricting image quality degradation of projection image.
2. Related Background Art
A known example of the spatial light modulator (light valve) conventionally used for projection type display apparatus is a spatial light modulator of a phase difference modulation type (polarization modulation type), which spatially modulating light, utilizing polarization. For example, a modulator formed using the liquid crystal (phase difference modulation type liquid crystal light valve) is practically available as the spatial light modulator of the phase difference modulation type.
The conventional projection type display apparatus using such a spatial light modulator of the phase difference modulation type uses a polarizing beam splitter (primary polarizing beam splitter) serving as a polarizer and an analyzer. For the below description, let us assume that the polarizing beam splitter has such a property as to reflect an s-polarized light component and transmit a p-polarized light component. In the conventional projection type display apparatus, the polarizing beam splitter splits light incident thereinto (light directly incident from an illumination light source or light after color-separated before incidence thereto) into light of the p-polarized light component and light of s-polarized light component, and normally, the s-polarized light component out of the light thus split into is projected to the spatial light modulator. After modulated and reflected by a liquid crystal layer of the spatial light modulator, the light again goes back into the polarizing beam splitter. On this occasion, the reflected light from the spatial light modulator 4 is analyzed by the polarizing beam splitter. The above polarizing beam splitter reflects the s-polarized light component, but transmits only the p-polarized light resulting from the modulation by the spatial light modulator. The transmitted light (that is, the analyzed light) is projected as a projection image through a projection optical system onto a screen or the like.
Many polarizing beam splitters and color separating/combining optical systems used in such projection type display apparatus have been proposed as liquid immersion type optical components in such structure that a plate of an optically transparent material coated with a coating for polarizing beam splitter and a plate of an optically transparent material coated with a color separating dichroic film were respectively immersed in a liquid with adjusted refractive index, as disclosed for example in U.S. Pat. No. 4,687,301 owned by Hughes Aircraft Co. The refractive index of the above liquid, at a predetermined temperature of the liquid, is adjusted so as to be equal to that of the plate of the transparent material. The reason why the beam splitter is immersed in the liquid is that, supposing the beam splitter were set in air, the interface of the coating would be in a relation of air against transparent material and the polarizing beam splitter would fail to function because of a difference in refractive index.
In the conventional projection type display apparatus, the light incident into the polarizing beam splitter is split by the polarizing beam splitter into the p-polarized light component and s-polarized light component, among which the s-polarized light component is projected to the color separating/combining optical system. After the color separating/combining optical system separates the incident s-polarized light component into some color components, the separated color components are respectively modulated by liquid crystal layers and respectively reflected by reflection layers of light valves which are prepared in accordance with the color components. The modified color components are combined by the color separating/combining optical system, and thereafter the combined light returns to and is analyzed by the polarizing beam splitter. The analyzed light is projected as a projection image through the projection optical system onto the screen or the like.
The inventors found out the following problems after investigation on the conventional projection type display apparatus employing the polarizing beam splitter and the color separating/combining optical system of the liquid immersion type as discussed above.
First, in the case of the above liquid immersion type polarizing beam splitter and the liquid immersion type color separating/combining optical system, a change of the refractive index of the liquid in which the plate of the transparent material is immersed depends upon a change of the temperature of the liquid. Namely, even with the liquid adjusted in its refractive index at a certain liquid temperature, the temperature change of the liquid itself will make a difference between the refractive index of the liquid and the refractive index of the plate of the transparent material. This changes the performances of the entire optical components. For example, in the case of a certain sample (the liquid for the above liquid immersion type optical components), the refractive index changes about 0.000349 per temperature rise of 1xc2x0 C., and this change rate is two order greater than those of substrate materials for plates of normal transparent materials. Normally, use environments (for example, temperatures) of the projection type display apparatus include a possibility of change of about 20xc2x0 C. to 60xc2x0 C., so that the difference in refractive index becomes unignorable. Since dispersion also changes, it causes chromatic aberration and chromatic unevenness in the projection image.
Second, in the case of the above liquid immersion type polarizing beam splitter and the liquid immersion type color separating/combining optical system, if the temperature change in the liquid is not even, the temperature dependence of the refractive index of the liquid as discussed above will affect the evenness of the refractive index of the liquid, thereby forming an index distribution in the liquid. In practical projection type display apparatus, the liquid temperature rarely changes evenly over the entire liquid (whereby the refractive index of the entire liquid is not even), which will be a great cause to damage the evenness of the projection image.
Third, in the case of the liquid immersion type polarizing beam splitter and the liquid immersion type color separating/combining optical system, the above uneven temperature change of the liquid destroys the evenness of the density of the liquid as well as that of the refractive index of the liquid as discussed above, resulting in causing convection in the liquid. Since this convection causes a time change of the uneven index distribution in the liquid as described above, the occurrence of convection will be a cause to change the unevenness of picture quality with time in the projection type display apparatus.
Fourth, in the above liquid immersion type polarizing beam splitter and the liquid immersion type color separating/combining optical system, the volume of the liquid itself also changes with a change of the liquid-temperature. In the case of the above sample, the temperature change of 1xc2x0 C. changes the volume at 0.00073 cc per cc. The use environments (for example, the temperatures) of the projection type display apparatus cover a temperature difference of about 40xc2x0 C., but, considering transportation and storage in warehouse, it is necessary to take account of the temperature range of approximately xe2x88x9210xc2x0 C. to 80xc2x0 C. Although the volume change itself of the liquid gives a small effect-on the projection image, some mechanism is needed for absorbing the volume change of the liquid because of the configuration of the apparatus.
Fifth, if there is dust in the liquid of the above liquid immersion type polarizing beam splitter and the liquid immersion type color separating/combining optical system, the projection type display apparatus employing the liquid immersion type optical components will indicate the dust in the liquid in the projection image enlarged some ten to some hundred times, even if the dust in the liquid is not located near the focal point. Considering this situation, there should exist no dust in the liquid. Accordingly, assembling of the above liquid immersion type polarizing beam splitter requires a clean room and a work for removing dust and foreign matter in the liquid therefrom.
Sixth, if a bubble exists in the liquid in the above liquid immersion type polarizing beam splitter and the liquid immersion type color separating/combining optical system, the bubble will appear in the projection image, and thus, it should be preliminarily removed.
Seventh, because the above liquid immersion type polarizing beam splitter and the liquid immersion type color separating/combining optical system use the liquid because of its structure, it is necessary to provide a case for housing the liquid with a means for preventing leakage of the liquid, such as an O-ring.
As discussed above, the liquid immersion type polarizing beam splitter and the liquid immersion type color separating/combining optical system have a lot of problems because of its structural feature or the like, and the projection type display apparatus employing it naturally requires a lot of time and labor for production thereof, which results in increasing the cost. Particularly, the characteristic changes of the refractive index etc. due to the liquid temperature change of the liquid are substantially unavoidable problems. Since the liquid immersion type polarizing beam splitter cannot be set at the setting angle of 45xc2x0 relative to the optical axis because of the refractive index, the projection type display apparatus employing the polarizing beam splitter becomes large and heavy.
Moreover, in the case of the conventional polarizing beam splitter and color separating/combining optical system each being constructed by a block of the transparent material, optical anisotropy of glass caused by various factors induces double refraction, which could disturb the optical characteristics of the optical components, possibly resulting in failing to fully reduce the image quality degradation of the projection image. Here, the various factors mainly include processing steps of the transparent material (cutting, bonding with another material, and film formation on surface), external stress caused in the operation of incorporating the transparent material into the optical system (holding with a jig, adhesion, etc.), thermal stress caused by heat generation inside the transparent material (absorption of light energy etc.) or external heat generation (heat generation of peripheral devices etc.), and stress caused when the transparent material is bonded in contact with another material of a different thermal expansion coefficient during heat generation. As described, these various thermal stress and external stress occurs throughout the period ranging from fabrication of the transparent material block and processing of the optical components to the operating duration of the projection type display apparatus, and it is thus very difficult to eliminate the all factors.
The present invention has been accomplished to solve the above problems, and an object of the invention is to provide a projection type display apparatus which employs a polarizing beam splitter and a color separating/combining optical system each being constructed of a transparent material block (solid material), thereby removing the various problems resulting from employment of the above-discussed liquid immersion type polarizing beam splitter, being capable of securing optically stable performance against the influence of various thermal stress and external stress in the transparent material block, and decreasing the degradation of image quality.
The projection type display apparatus according to the present invention is a display apparatus for effecting, at least, color separation, polarized light separation, and color combination with respect to light from a light source. The apparatus mainly encompasses a configuration in which, after color separation, each light component is subjected to polarized light separation; and a configuration in which, after polarized light separation, thus separated light beam is divided into individual color light components.
First, in the configuration in which each light component obtained after color separation is subjected to polarized light separation, the projection type display apparatus according to the present invention comprises, at least, a color separating optical system for separating light from-a light source into red, green, and blue light components; a polarized light separating optical system for separating an incident light component into polarized light components different from each other; a light valve (spatial light modulator) for modulating a light component; a color combining optical system for combining a plurality of incident light components in terms of color; and a projection optical system for projecting light resulting from color combination effected by the color combining optical system onto a predetermined screen or the like.
Also, the projection type display apparatus according to the present invention comprises a first optical system disposed so as to correspond to the red light component resulting from color separation effected by the color separating optical system, a second optical system disposed so as to correspond to the green light component, and a third optical system disposed so as to correspond to the blue light component. Here, the blue light component (hereinafter referred to as xe2x80x9cB-light componentxe2x80x9d) refers to light in a wavelength range of 380 to 500 nm, the green light component (hereinafter referred to as xe2x80x9cG-light componentxe2x80x9d) refers to light in a wavelength range of 500 to 600 nm, and the red light component (hereinafter referred to as xe2x80x9cR-light componentxe2x80x9d) refers to light in a wavelength range of 600 to 700 nm.
The first optical system comprises a first polarized light separating optical system for emitting a first polarized light component according to the R-light component incident thereon; a first light valve for modulating the first polarized light component emitted from the first polarized light separating optical system; and a first analyzing optical system for analyzing the first polarized light component modulated by the first light valve. The second optical system comprises a second polarized light separating optical system for emitting a second polarized light component according to the G-light component incident thereon; a second light valve for modulating the second polarized light component emitted from the second polarized light separating optical system; and a second analyzing optical system for analyzing the second polarized light component modulated by the second light valve. The third optical system comprises a third polarized light separating optical system for emitting a third polarized light component according to the B-light component incident thereon; a third light valve for modulating the third polarized light component emitted from the third polarized light separating optical system; and a third analyzing optical system for analyzing the third polarized light component modulated by the third light valve.
Here, in the projection type display apparatus according to the present invention, the first to third polarized light separating optical systems may be constituted by first to third polarizing beam splitters, respectively. Also, the first to third analyzing optical systems may be constituted by the first to third polarizing beam splitters, respectively. Further, the apparatus may be configured such that the pairs of the polarized light separating optical system and analyzing optical system for the R-light component, the polarized light separating optical system and analyzing optical system for the G-light component, and the polarized light separating optical system and analyzing optical system for the B-light component commonly use their corresponding polarizing beam splitters (first to third polarizing beam splitters) provided for the respective light components.
In particular, in the above-mentioned configuration, the optical member constituting the polarizing beam splitters is preferably made of an optically transparent material in which a wavelength where the absolute value of its photoelastic constant becomes a minimum level exists in the wavelength range of at least one of the B-light component and G-light component. Alternatively, this optical member is preferably made of an optically transparent material in which, among a first value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the R-light component, a second value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the G-light component, and a third value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the B-light component, the second or third value is the smallest. Specifically, the optically transparent material is preferably a material in which the photoelastic constant with respect to each wavelength in the wavelength range of the R-light component, at least, is not greater than +1.5xc3x9710xe2x88x928cm2/N.
On the other hand, in the configuration in which color separation is effected after light from a light source is subjected to polarized light separation, the projection type display apparatus according to the present invention comprises, at least, a polarized light separating optical system for separating the light from the light source into polarized light components different from each other; a color separating optical system for separating a predetermined polarized light component incident thereon into red, green, and blue light components; a light valve for modulating a light component incident thereon; a color combining optical system for combining a plurality of incident light components in terms of color; and a projection optical system for projecting light resulting from color combination effected by the color combining optical system onto a predetermined screen or the like.
Also, the projection type display apparatus according to the present invention comprises a first optical system disposed so as to correspond to the red light component resulting from color separation effected by the color separating optical system, a second optical system disposed so as to correspond to the green light component, and a third optical system disposed so as to correspond to the blue light component.
The first optical system comprises a first light valve for modulating the R-light component incident thereon, and a first analyzing optical system for analyzing the polarized light component modulated by the first light valve. The second optical system comprises a second light valve for modulating the G-light component incident thereon, and a second analyzing optical system for analyzing the polarized light component modulated by the second light valve. The third optical system comprises a third light valve for modulating the B-light component incident thereon, and a third analyzing optical system for analyzing the polarized light component modulated by the third light valve.
Here, in the projection type display apparatus according to the present invention, the polarized light separating optical system may be constituted by a polarizing beam splitter. In this configuration, in particular, the optical member constituting the polarizing beam splitter is preferably made of an optically transparent material in which a wavelength where the absolute value of its photoelastic constant becomes a minimum level exists in the wavelength range of at least one of the B-light component and the G-light component. Alternatively, this optical member is preferably made of an optically transparent material in which, among a first value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the R-light component, a second value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the G-light component, and a third value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the B-light component, the second or third value is the smallest. Specifically, the optically transparent material is preferably a material in which the photoelastic constant with respect to each wavelength in the wavelength range of the R-light component, at least, is not greater than +1.5xc3x9710xe2x88x928cm2/N.
Also, the first to third analyzing optical systems may be constituted by first to third polarizing beam splitters, respectively. In this configuration, in particular, the optical member constituting the polarizing beam splitters is preferably made of an optically transparent material in which a wavelength where the absolute value of its photoelastic constant becomes a minimum level exists in the wavelength range of at least one of the B-light component and the G-light component. Alternatively, this optical member is preferably made of an optically transparent material in which, among a first value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the R-light component, a second value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the G-light component, and a third value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the B-light component, the second or third value is the smallest. Specifically, the optically transparent material is preferably a material in which the photoelastic constant with respect to each wavelength in the wavelength range of the R-light component, at least, is not greater than +1.5xc3x9710xe2x88x928cm2/N.
Additionally, the color separating optical system and the color combining optical system are preferably constituted by a common optical system. The common optical system is constituted by a plurality of prism assemblies, and the each of the plurality of prism assemblies is preferably made of an optically transparent material in which a wavelength where the absolute value of its photoelastic constant becomes a minimum level exists in the wavelength range of at least one of the B-light component and the G-light component. Alternatively, each of the plurality of prism assemblies is preferably made of an optically transparent material in which, among a first value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the R-light component, a second value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the G-light component, and a third value which is a mean value of absolute values of photoelastic constants with respect to individual wavelengths in the wavelength range of the B-light component, the second or third value is the smallest. Specifically, the optically transparent material is preferably a material in which the photoelastic constant with respect to each wavelength in the wavelength range of the R-light component, at least, is not greater than +1.5xc3x9710xe2x88x928cm2/N.
The inventors have discovered that the photoelastic constant of an optically transparent material such as glass has a characteristic, as a function of wavelength, which is upward to the right (the longer the wavelength is, the higher becomes the photoelastic constant) while yielding an upward convex profile. The inventors have also discovered that, by changing the composition of the optically transparent material, the wavelength at which the absolute value of photoelastic constant becomes a minimum level can be changed while such a characteristic is maintained. Further, the smaller the absolute value of photoelastic constant is, the less occurs birefringence with respect to influences of various thermal and external stresses, whereby optically stable performances can be secured. Accordingly, when a polarizing beam splitter constituted by an optically transparent material having a small absolute value of photoelastic constant is used in a projection type display apparatus, the image quality of the projected image can be restrained from deteriorating. Here, the inventors have discovered that, in order to restrain the image quality from deteriorating, while the absolute value of photoelastic constant is preferably made smaller as the wavelength of light decreases, it is not necessary for the absolute value of photoelastic constant to be so small with respect to light having a long wavelength. It is due to the fact that, while an optically transparent material absorbs light and generates heat, thereby expanding itself according to its coefficient of linear expansion and generating an internal stress, such absorption becomes smaller as the wavelength of light is longer, whereby the longer the wavelength of light is, the less occurs the internal stress upon light absorption.
The present invention is based on these new discoveries achieved by the inventors.
The above-mentioned projection type display apparatus according to the present invention employs a polarizing beam splitter constituted by an optical member made of an optically transparent material, without using the above-mentioned conventional liquid immersion type polarizing beam splitter. Accordingly, the present invention can eliminate various problems accompanying the use of the above-mentioned conventional liquid immersion type polarizing beam splitter, thus advantageously facilitating the manufacture of the apparatus, for example.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.